This invention relates to a process for the manufacture of dimethylaluminum chloride, a compound known to be useful in a variety of applications. Common examples of these applications are use as a cocatalyst with transition metal compounds for the polymerization of olefins and dienes, as a methylating agent, and as a starting material for the manufacture of trimethylaluminum, a compound of considerable utility as a cocatalyst, chemical intermediate, and hypergolic fuel.
One of the known processes for the manufacture of dimethylaluminum chloride involves the reaction of methyl chloride with a special aluminum-magnesium alloy, Al.sub.2 Mg; C. J. Marsel, E. O. Kalil, A. Reidlinger, and L. Kramer, Advances in Chemistry Series, No. 23, p. 176 (1959): EQU 4CH.sub.3 Cl + Al.sub.2 Mg .fwdarw. 2(CH.sub.3).sub.2 AlCl + MgCl.sub.2
This process has the disadvantage of a high raw material cost, since the cost of the alloy is significantly higher than the cost of aluminum. Furthermore, magnesium chloride is formed as a by-product and presents disposal problems because of its physical properties and low commercial value. The use of a 70 weight % aluminum/30 weight % magnesium alloy, commercially produced by Dow Chemical Co., is also described in the reference. Unfortunately, the aluminum content of the 70/30 alloy is higher than that of Al.sub.2 Mg, which contains only 69 weight % aluminum. Thus, the dimethylaluminum chloride produced therefrom is contaminated with a small amount of methylaluminum dichloride.
U.S. Pat. No. 2,786,860 teaches a process for the production of organic aluminum compounds by reacting an aluminum halide or alkylaluminum halide with an alkali hydride and an olefin having a terminal double bond. This process involves a three-step reaction cycle, which must be repeated several times to obtain the desired yield, while recycling one of the by-products as a starting material.
Dimethylaluminum chloride can also be produced by reaction between trimethylaluminum and methylaluminum sesquichloride. The latter is an equimolar mixture of dimethylaluminum chloride and methylaluminum dichloride. This process also suffers from a high raw materials cost, in this case due to trimethylaluminum.
Numerous reactions are known in the art to produce alkylaluminum sesquihalides. U.S. Pat. No. 2,863,894 teaches the reaction between aluminum and a primary alkyl halide in the presence of an inert solvent. When the sesquihalide is produced, the alkylaluminum dihalide is present in 50% molar proportion, and must be separated by distillation.
Dimethylaluminum chloride is also produced by reaction of methylaluminum sesquichloride with sodium chloride (V. F. Hnizda and C. A. Kraus, J. Amer. Chem. Soc. 60, p. 2276 (1938). In this process, over half of the methylaluminum sesquichloride is converted to CH.sub.3 AlCl.sub.2.NaCl, a complex salt of very little commercial utility which presents disposal problems because of its low solubility in organic solvents and the potential hazards due to its vigorous reactivity toward aqueous solvents.